Mineral fertilizers are a cornerstone of modern day agriculture. Without them, worldwide production of all crops would be greatly reduced, placing a greater stress on our food supply system.
Fertilizers come in two basic forms, liquid and dry. In the U.S., over the last 50 years, liquid fertilizers have been gaining in popularity mainly due to the ease of handling and application.
For a liquid fertilizer to become a commercially viable product, it must have a fairly high nutrient analysis and must be able to blend with other liquid fertilizers to supply the required nutrients for a growing crop.
Urea ammonium nitrate (UAN) is a popular liquid fertilizer containing about 28-32% nitrogen. Ammonium polyphosphate (APP), another commonly used liquid fertilizer, contains from about 34-47% phosphate.
Sulfur (S) has emerged as a major nutrient for plants. It is considered the fourth major plant nutrient, after nitrogen, phosphorous and potassium, due to the amount required by plants. Sulfur is essential as a structural component of some amino acids found in both plants and animals, and is a part of the makeup of every living organism. Chlorophyll formation is also dependent on proper sulfur uptake.
About 10 million metric tons of sulfur-containing fertilizers are applied annually worldwide and have the potential to increase another 8 million tons. Applications of sulfur-containing fertilizers will increase even more due to the reduction in industrial sulfur dioxide emissions and the subsequent depletion of sulfur in the soil. In recent history, industrial sulfur dioxide emissions captured and transported by rainfall have been a significant source of sulfur for crop production, but environmental restrictions have reduced airborne SO2 levels since the 1980's. Soil sulfur levels have become depleted with prolonged crop removal, sulfate leaching, low precipitation deposition, and declining soil organic matter.
Although sulfur exists in many different chemical forms, plants can only absorb sulfur through their root systems in the form of sulfate anion (SO4−2). Small amounts of sulfur dioxide gas can be absorbed through the plant's leaves, but the amount is too small to satisfy the plant's need.
There are two types of available sulfur-containing fertilizers for the crop application. The first type includes those fertilizers that are in the form of sulfate, which are ready for the plant's uptake. Examples of sulfate fertilizers are ammonium sulfate, sulfate of potash, and single superphosphate (SSP). These fertilizers may contain nitrogen (N), phosphorous (P), or potassium (K) as well, but they also are important due to sulfur (S) content. Sulfate fertilizers represent about 75% of the sulfur fertilizers applied annually.
The second type of sulfur-containing fertilizers are those materials containing sulfur in oxidation states of the sulfur other than sulfate, which need to go through chemical oxidation reaction to form the sulfate form which can be taken up by plants. Thiosulfate and elemental sulfur are examples of the second type. These forms of sulfur undergo chemical oxidation in the environment to convert to sulfate anion.
Another form of sulfur can exist as sulfite anion (SO3−2) or as bisulfite anion (HSO3−1). Potassium sulfite and potassium bisulfite are examples of sulfur in the sulfite or bisulfite forms. Potassium sulfite has been used as an anti-browning agent, an antioxidant and as a preservative. It is used as wine, beer, and fruit juice preservation. It is also used in fresh fruit and meat preservation. The preservative potassium sulfite is used as an E225 preservative. Both potassium bisulfite and potassium sulfite are used as photographic chemicals in film development.
In addition to being characterized according to their content and form, fertilizers are also characterized by how they are utilized in horticulture. For example, “starter fertilizer” is used to promote the growth of newly planted crops, particularly newly germinated seeds. Starter fertilizers are applied in low doses close to the plant seed to meet the demands of the seedling for nutrients until the plant's root system develops. They also enhance the development of the emerging seedling. Starter fertilizers are most beneficial when the crops are planted in cold and wet soil in the early spring or late fall. They are also used when the soil nutrients levels are low. In addition to N, P, and K, sulfur is a key component of the starter fertilizer.
To improve early season plant growth in cool soils, many crop producers band small amounts of starter fertilizers at planting. Applying fertilizer 2 inches to the side of the seed row and 2 inches below the seed row (2×2) or 2 inches to the side and on the surface (2×0) places the nutrients in a good position for root absorption. This has proven to be very effective for many crops, especially in minimum and no-tillage conditions where soils remain cooler for a longer period of time in the early spring.
To accomplish this, separate fertilizer openers are required to place the material in a 2×2 placement. This has a number of disadvantages such as cost, weight of the openers, residue clearance, planter space and soil disturbance which can adversely affect seed placement(1).
To avoid these problems, growers are placing fertilizers directly with the seed. This type of application is called a pop-up or in-furrow treatment. In-furrow treatments have proven to be agronomically as effective as 2×2 and 2×0 placements(2). However, there are several factors to consider that can have a negative effect on germination and/or seedling injury.
Crop sensitivity to fertilizer salts, cation exchange capacity (CEC) of the soil, soil type and moisture, organic matter, row spacing, and fertilizer band width are a few factors that must be considered in choosing the fertilizer and method for starter fertilization, along with the amount of risk a grower is willing to accept, including possible crop maturity delays(3).
Germination damage caused by fertilizers is primarily due to salt or osmotic effect, where fertilizer salts draw critical moisture from the seeds and soil surrounding the seeds. In some cases there is a toxic ion effect where certain ions can be toxic to germinating seeds, such as ammonia generated from urea hydrolysis. It is for this reason that university agricultural extension personnel do not recommend banding urea with the seed.
Many crops require side dressing to flourish properly. The term “top-dress” usually refers to broadcast applications on crops like small grains. The term “side dress” refers to fertilizer placed at relatively high amounts anywhere from three to four inches from the row to half way between the crop rows.
Plants absorb nutrients as well as other chemicals through their foliage to varying degrees. Growers in most all types of agriculture apply foliar nutritional sprays from time to time for various reasons. A basic philosophy many growers utilize is to apply what is believed to be required to the soil in the fertilization program, and use nutritional foliar supplements as a tool to give crops any nutrients they may still be lacking. Even though growers and researchers use this technique as a nutritional supplement, the mechanism of foliar absorption of nutrients is not well understood.